Hi
1. It depends, if the child requires a TRV (visitor visa) then you may find that the overseas office will not issue one nor a Temporary Resident Permit for the child. If so then one of the parents will have to return to Canada to submit a sponsorship for the child.
2. If the child is from a visa exempt country, then it is possible to return to Canada with the child, and then submit a sponsorship application, using either the country of citizenship office for the processing, or if s/he is admitted for a year, then you could also use Buffalo.